The present invention relates to magnetic disc drive data storage devices, and more particularly to rotary actuators employed in such devices to selectively position magnetic transducing heads relative to rotating discs.
Magnetic disc drive devices generally employ either linear or rotary actuators to position magnetic reading and recording heads with respect to rotatable magnetic discs. Such heads typically are moved at least generally radially of the discs in order to locate a head proximate a desired track on the disc.
Rotary actuators usually include a shaft mounted rotatably on the deck or other stationary housing portion of the drive, along with an arm or group of stacked arms connected to the shaft for pivoting with the shaft. A magnetic transducing head is mounted to the end of each arm remote from the shaft, and thus is carried in an arcuate path as the shaft pivots. Usually, multiple head support arms are mounted to the shaft in stacked relation, for rotation in concert with the shaft.
The prevailing means for controllably pivoting the head arm assembly is an electric motor including a voice coil and permanent magnets. Typically the voice coil is a flat loop perpendicular to the shaft pivot axis, connected to the shaft for pivoting with the shaft. The permanent magnets then are integral with the disc drive housing, positioned in spaced apart relation to the voice coil and on opposite sides of the coil. When electrical current is directed through the wires of the voice coil, a magnetic field is generated which interacts with the magnetic field of the permanent magnet to provide the force which moves the voice coil and rotates the shaft as well.
This orientation of the voice coil, however, gives rise to a problem in that only the radially outward portion of the voice coil has a desirably long moment arm, i.e. distance from the pivot axis. The radially inward portions of the coil have shorter effective moment arms, and thus provide correspondingly reduced rotational force on the shaft and head arm assembly. Of course, this difficulty can be counteracted by increasing the size of the voice coil, yet such a "solution" runs contrary to the continuing trend toward smaller and more compact disc drive components, including rotary actuators. Large voice coils further are a disadvantage in that they add to the inertia of the shaft and head arm assembly, resulting in a head arm assembly which cannot be accelerated and decelerated as rapidly as desired, or conversely require increased power to accelerate and decelerate the assembly.
Another challenge associated with rotary actuators is the desire to employ lightweight materials, e.g. aluminum, to the extent possible to further reduce inertia of the head arm assembly, as well as reducing overall weight. At the same time, other necessary structure, e.g. flux carrying bodies constructed of steel, have different thermal expansion coefficients and thus raise the potential for distortion due to the varying thermal coefficients of contiguous structures in the actuator.
Therefore, it is an object of the present invention to provide a rotary actuator in which substantially the entire voice coil is located at a constant radial separation from the rotary actuator pivot shaft.
Another object is to provide a voice coil curved about the pivot axis of a rotary actuator, and having a thickness dimension radially of the pivot axis selected to facilitate rapid dissipation of heat from the voice coil.
A further object is to provide a voice coil of reduced mass for reduced inertia of a head arm assembly of a rotary actuator, at no reduction in available power to accelerate and decelerate the head arm assembly.
Yet another object is to provide a rotary actuator including means to compensate for different thermal expansion coefficients in various materials employed in constructing the actuator.